Number 1 in FIG. 1 indicates as a whole a helicopter substantially comprising a fuselage 2 housing the crew and on-board equipment; and a rotor 3 projecting from a top portion of fuselage 2 and for generating a force by which to sustain helicopter 1.
In more detail, on the opposite side to rotor 3, fuselage 2 is bounded by a deck 4 for supporting the crew and on-board equipment.
More specifically, deck 4 is of known type, comprises a frame 5 for imparting to deck 4 the necessary structural strength to support the crew and on-board equipment, and defines a crew tread surface 6 on one side of frame 5, and, on the opposite side, a portion 7 of the outer surface of fuselage 2.
Frame 5, shown partly in FIGS. 2 and 3, comprises a lattice defined by a number of longitudinal members 10 (only one shown) extending in respective parallel directions A, and by a number of cross members 11 (only one shown) extending in respective parallel directions B intersecting directions A perpendicularly at a number of nodes 12 (only one shown).
Cross members 11 are interrupted at each node 12 to avoid interfering with longitudinal members 10, and are fixed to longitudinal members 10, at each node 12, by an anchoring device 13 (only one shown in detail).
More specifically, longitudinal members 10 are defined by elongated flat walls, and are bounded, on opposite sides of direction A, by respective surfaces 21, 22. Similarly, cross members 11 are defined by elongated flat walls, interrupted at the various nodes 12, and are bounded, on opposite sides of direction B, by respective surfaces 17, 18.
More specifically, anchoring device 13 comprises two pairs 27, 28 of connecting members 14, which are located on opposite sides of direction B to connect respective surfaces 17 and 18 to surfaces 21 and 22.
More specifically, connecting members 14 in pair 27 are located on opposite sides of direction A; a first connecting member 14 connects surface 17 to surface 21; and a second connecting member 14 connects surface 17 to surface 22.
Similarly, connecting members 14 in pair 28 are located on opposite sides of direction A; a first connecting member 14 connects surface 18 to surface 21; and a second connecting member 14 connects surface 18 to surface 22.
More specifically, connecting members 14 are identical, and each comprise a portion 29 and a portion 30, which are of equal extension, are perpendicular to each other, are fixed to longitudinal member 10 and cross member 11 respectively, are parallel to directions A and B respectively, and are therefore joined at node 12.
FIG. 4 shows a graph of the force F exerted at a given node 12 versus the displacement s of a point on deck 4 corresponding to given node 12, in the event of impact in a direction perpendicular to deck 4.
The graph comprises a portion 31 increasing steadily to a maximum value corresponding to reversible elastic deformation of anchoring device 13; and a decreasing portion 32 following portion 31 and corresponding to permanent global deformation of anchoring device 13.
More specifically, along portion 31, the energy acquired by deck 4 during impact is transmitted back to tread surface 6, and from tread surface 6 to the crew and on-board equipment.
Along portion 32, on the other hand, as opposed to being transmitted to tread surface 6, the energy acquired by deck 4 during impact is dissipated in the form of permanent deformation of deck 4.
Deck 4 is therefore only able to dissipate energy in the form of permanent deformation after transmitting the maximum force to tread surface 6, which may correspond to an intolerable amount of energy transmitted to the crew and passengers.
A need is therefore felt in the industry to reduce the amount of energy transmitted by the deck to the tread surface, in the event of impact, to safeguard the crew and passengers.
Moreover, for certain missions, helicopter certification regulations require that the helicopter deck be capable of absorbing a given amount of impact energy, without transmitting it to the crew or passengers.